Distributed Arithmetic: Implementations and Applications

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Distributed Arithmetic: Implementations and Applications. A Tutorial. Distributed Arithmetic (DA) [ Peled and Liu ,1974]. An efficient technique for calculation of sum of products or vector dot product or inner product or multiply and accumulate (MAC)

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### Distributed Arithmetic: Implementations and Applications

A Tutorial

Distributed Arithmetic (DA) [Peled and Liu,1974]
• An efficient technique for calculation of sum of products or vector dot product or inner product or multiply and accumulate (MAC)
• MAC operation is very common in all Digital Signal Processing Algorithms
So Why Use DA?
• The advantages of DA are best exploited in data-path circuit designing
• Area savings from using DA can be up to 80% and seldom less than 50% in digital signal processing hardware designs
• An old technique that has been revived by the wide spread use of Field Programmable Gate Arrays (FPGAs) for Digital Signal Processing (DSP)
• DA efficiently implements the MAC using basic building blocks (Look Up Tables) in FPGAs
An Illustration of MAC Operation
• The following expression represents a multiply and accumulate operation
• A numerical example
A Few Points about the MAC
• Consider this

Note a few points

• A=[A1, A2,…, AK] is a matrix of “constant” values
• x=[x1, x2,…, xK] is matrix of input “variables”
• Each Ak is of M-bits
• Each xk is of N-bits
• y should be able large enough to accommodate the result
A Possible Hardware (NOT DA Yet!!!)
• Let,

Shift right

Registers to hold sum of partial products

Multi-bit AND gate

Each scaling accumulator calculates Ai X xi

Shift registers

How does DA work?
• The “basic” DA technique is bit-serial in nature
• DA is basically a bit-level rearrangement of the multiply and accumulate operation
• DA hides the explicit multiplications by ROM look-ups an efficient technique to implement on Field Programmable Gate Arrays (FPGAs)
Moving Closer to Distributed Arithmetic

…(1)

• Consider once again
• a. Let xk be a N-bits scaled two’s complement number i.e.

| xk | < 1

xk : {bk0, bk1, bk2……, bk(N-1) }

where bk0 is the sign bit

• b. We can express xk as
• c. Substituting (2) in (1),

…(2)

…(3)

Moving More Closer to DA

…(3)

Expanding this part

Almost there!

…(4)

The Final Reformulation

Lets See the change of hardware

Our Original Equation

Bit Level Rearrangement

So where does the ROM come in?

Note this portion. It’s can be treated as function of serial inputs bits of

{A, B, C,D}

The ROM Construction
• has only 2K possible values i.e.
• (5) can be pre-calculated for all possible values of b1n b2n …bKn
• We can store these in a look-up table of 2Kwordsaddressed byK-bits i.e. b1n b2n …bKn

…(4)

…(5)

Lets See An Example
• Let number of taps K=4
• The fixed coefficients are A1 =0.72, A2= -0.3, A3 = 0.95, A4 = 0.11
• We need 2K = 24 = 16-words ROM

…(4)

Key Issue: ROM Size
• The size of ROM is very important for high speed implementation as well as area efficiency
• The number of address lines are equal to the number of elements in the vector i.e. K
• Elements up to 16 and more are common => 216=64K of ROM!!!
• We have to reduce the size of ROM
A Very Neat Trick:

…(6)

2‘s-complement

…(7)

Re-Writing xk in a Different Code
• Define: Offset Code
• Finally

…(7)

…(8)

Using the New xk
• Substitute the new xk in here

…(9)

Hardware Using Offset Coding

x1 selects between the two symmetric halves

Ts indicates when the sign bit arrives

Alternate Technique: Decomposing the ROM

Speed Concerns
• We considered One Bit At A Time (1 BAAT)
• No. of Clock Cycles Required = N
• If K=N, then essentially we are taking 1 cycle per dot product  Not bad!
• Opportunity for parallelism exists but at a cost of more hardware
• We could have 2 BAAT or up to N BAAT in the extreme case
• N BAAT  One complete result/cycle
The Speed Limit: Carry Propagation
• The speed in the critical path is limited by the width of the carry propagation
• Speed can be improved upon by using techniques to limit the carry propagation
Speeding Up Further: Using RNS+DA
• By Using RNS, the computations can be broken down into smaller elements which can be executed in parallel
• Since we are operating on smaller arguments, the carry propagation is naturally limited
• So by using RNS+DA, greater speed benefits can be attained, specially for higher precision calculations
Conclusion
• Ref: Stanley A. White, “Applications of Distributed Arithmetic to Digital Signal Processing: A Tutorial Review,” IEEE ASSP Magazine, July, 1989
• Ref: Xilinx App Note, ”The Role of Distributed Arithmetic In FPGA Based Signal Processing’